Nanjing Underwater World is the main aquarium in the Purple Mountain scenic area, close to several of the city's biggest sights. It is not the reason to come to Nanjing, but it is a practical and enjoyable break if you are travelling with children or need an indoor stop between tombs, temples, and long walks through the parkland.
The appeal is straightforward: tanks of marine life, underwater tunnel views, and scheduled animal shows in a setting that is easy to combine with Sun Yat-sen Mausoleum or Ming Xiaoling. The surrounding area is one of Nanjing's strongest sightseeing districts, so this works best as part of a broader Purple Mountain day rather than a stand-alone excursion.